1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus forming an image by discharging droplets.
2. Discussion of the Related Art
An image forming apparatus such as a printer, a facsimile machine, or a multifunctional apparatus including the above functions further includes an inkjet-type image forming apparatus. The inkjet-type image forming apparatus includes a droplet discharging device including a recording head. The recording head is formed of a liquid discharging head, which discharges droplets of recording liquid (or ink) to be attached to a recording medium while conveying the recording medium, thereby forming images thereon. It is to be noted that terms such as recording, printing, print, and the like may also be used for image formation.
A well-known image forming apparatus including a liquid discharging device is in general configured as follows. A small capacity liquid container or a head tank (which may also be called a sub tank) which supplies liquid (hereinafter, to be referred to as ink) to the recording head on a carriage is provided. The image forming apparatus includes a main cartridge or a main tank in the apparatus body side and ink is supplied from the main cartridge of the apparatus body side to the head tank, or otherwise, an ink cartridge, a replaceable liquid container, is provided together with the recording head.
The image forming apparatus including a commonly used head tank has an ink supply path, to supply ink from the main tank to the head tank, formed of in many cases a flexible resinous tube whose precise configuration and dimensions are determined by the layout, ease of assembly, and ease of maintenance. If negative pressure generation means is to be provided inside the head tank, a flexible film is used for that means in many cases. The tube and film, however, are susceptible to being penetrated by air from outside gradually, and inevitably air gets into the head tank. Further, the air present in the main tank, mixed-in air in the supply path at a time of attachment/detachment of the main tank, and the air dissolved in the ink finally accumulate inside the head tank via the ink supply path.
To cope with the above problem, JP-2008-213392-A discloses a technique to securely discharge mixed-in air from an air discharging means provided to the sub tank. Specifically, a liquid moving means capable of sending ink to the sub tank which supplies ink from the main tank to the liquid discharging head and capable of returning ink from the sub tank to the main tank is provided. Due to this recycling of, the ink is not wasted and the mixed-in air may be securely discharged from an air discharging means provided to the sub tank.
The same also discloses that a recording head is disposed such that a nozzle to discharge ink faces the recording target and thus, the recording head needs to keep negative pressure so that the nozzle which is not expected to discharge ink does not leak any ink. As a means to keep the recording head negatively pressurized, there is a method to generate negative pressure by providing the recording head and the ink cartridge at positions having a positional difference in height from each other using the resultant water head difference.
In addition, Japanese Patent No. 4151939 (JP-4151939-B) discloses a technique to generate negative pressure inside the liquid container, in which a liquid container formed of a flexible member and a member biasing the liquid container toward outside are arranged; the volume of the space inside the liquid container is reduced by a pressurizing means and the liquid is supplied from an ink cartridge to the liquid container; and then, the biasing pressure by the biasing means is released and the negative pressure is formed inside the liquid container.
There is also another technique to generate negative pressure, in which a sub tank is formed of a flexible member and a member to press the sub tank toward outside, and a liquid moving means provided in the liquid supply path between a main tank and the above sub tank returns the ink from the sub tank to the main tank, thereby forming negative pressure.
The image forming apparatus of a general inkjet method employs ink as a consumable product, and in many cases, for ease of the replacement, employs a replaceable cartridge-type ink tank which can be attached to the inkjet recording apparatus. This cartridge-type ink tank (hereinafter “ink cartridge”) is replaced with a new ink cartridge if the ink therein is consumed and the ink cartridge becomes empty.
There are four colors of ink, yellow (Y), magenta (M), cyan (C) and black (K). Even though the general name of the color is the same, various types are available depending on the usage such as for photographic image printing, text printing, and the like. If another type of ink is to be used depending on the purpose, the current ink cartridge which is not empty is detached from the apparatus in the middle of the operation and another type of ink cartridge is attached for usage. This replacement work is repeatedly performed.
JP-H04-214360-A and JP-2006-281588-A disclose a type of ink cartridge in which the ink is contained in a small bag member having low air permeability for shielding the ink from outside air to prevent modification of quality of the ink, and this bag member is contained in a casing.
This bag-like ink cartridge includes an ink outlet through which ink is supplied to the recording head. This ink outlet is formed, for example, of a rubber seal. An ink supply needle provided to the inkjet recording device is penetrated through the rubber seal, thereby forming a penetration opening to reach an inside of the ink bag.
The ink inside the ink bag is sucked toward the ink supply needle due to a sucking force generated by the recording head. As the ink is sucked and sent to the recording head, the ink bag is depressurized corresponding to the consumed amount of ink and has negative pressure relative to the atmospheric pressure. Accordingly, if the ink cartridge is removed from the inkjet recording device at this moment, upon the ink supply needle is extracted from the penetration opening, outside air or dusts flow into the ink bag through the penetration opening. Moreover, the ink cartridge in the middle of the operation has a reduced amount of ink and therefore has a small size. Accordingly, the ink bag is not stable inside the casing during the replacement operation and the pressure inside the ink bag is turbulent, which may raise a risk that the outside air or the dusts flow into the ink bag. The mixing of the air and dust with the ink may cause a defective ink discharge of the recording head.
As aforementioned, mixing of air into the sub tank, the recording head, and the liquid supply path, and mixing of the outside air and dust into the ink cartridge both cause defective image formation. In order to prevent both from happening, the technique disclosed by JP-2008-213392, that is, a configuration to perform ink supply and return between the main tank and the sub tank (a liquid moving means), and the technique disclosed by Japanese Patent No. 4151939, that is, a configuration to use a backflow prevention valve for preventing outside air and dust from flowing into the ink cartridge, may be collaterally applied to construct an apparatus. However, the combined use of these techniques has a problem in that, when the liquid moving means returns the ink from the sub tank to the main tank, the backflow prevention valve operates and prevents the ink from returning from the sub tank to the main tank. Accordingly, a satisfactory amount of ink may not be returned from the sub tank to the main tank, and therefore, there may be a case in which the air bubbles are not extracted satisfactorily from the sub tank, the recording head and the liquid supply path.
There is also such an adverse effect that even though a part of the liquid supply path is shielded by the backflow prevention valve, when the liquid moving means sends a liquid from the sub tank to the main tank, the pressure of the liquid in the liquid supply path rises abnormally, resulting in a leak of the ink from joint parts of the liquid supply path or the smearing of the components inside the image forming apparatus, the ink cartridge, and the like with ink. In the worst case, the joint parts of the liquid supply path are broken and the ink leaks continuously while sending ink from the first liquid container to the second liquid container.
JP-H04-214360-A discloses an ink cartridge provided with a ball-shaped backflow prevention valve and a reverse flow method in which air is sent when performing a recording head dischargeability recovery operation so that defective discharge generating factors such as air bubbles, dusts and agglomerated ink remaining around the head common liquid chamber are discharged. The same also discloses that the reverse flow is received by the sub tank, but does not disclose a backflow prevention valve provided at the side of the image forming apparatus body, a buffer tank of the cartridge side, nor a normal close valve.
Because of these reasons, a need exists for an image forming apparatus capable of solving the aforementioned conventional problems.